This invention relates generally to an apparatus and control system for working sheet material and more specifically to a machine, control system and method for laser welding employing a tilting mechanism and a seam tracking device.
It is common to employ a welding or cutting head in combination with an articulating robot arm or a moving gantry. However, most articulating robots suffer from a lack of precision and stability due to their inherent heavy moment arms extending a significant distance from their stationary bases. Furthermore, articulating robots usually only have a static movement accuracy of +/-100 microns, at best. This lack of accuracy and lack of stability detrimentally affects welding and cutting precision of a head mounted on the arm's end. In contrast, gantries tend to be more stable and thus more accurate than articulating robots, but usually require significantly expensive concrete reinforcement within the gantry and difficult to achieve bridge machining tolerances. Notwithstanding, the gantry bridges are still somewhat imprecise due to machining tolerances.
To account for these tolerance and accuracy variations, optical seam tracking cameras and sensors have been recently used for various welding processes including gas metal arc welding, gas tungsten arc welding, plasma arc welding, submerged arc welding, flux-cord arc welding and laser beam welding. One such system is produced by Servo-Robot Inc. of Boucherville, Quebec, Canada. Other examples of such three dimensional vision seam sensing systems are disclosed in the following U.S. Pat. Nos. 5,168,141 entitled "Vision Guided Laser Welding" which issued to Tashjian et al. on Dec. 1, 1992; 4,969,108 entitled "Vision Seam Tracking Method and Apparatus for a Manipulator" which issued to Webb et al. on Nov. 6, 1990; and 4,621,185 entitled "Adaptive Welding Apparatus having Fill Control Correction for Curvilinear Weld Grooves" which issued to Brown on Nov. 4, 1986; all of which are incorporated by referenced herewithin. Most welding and cutting gantry devices employing optical sensing use the somewhat imprecise bridge as the Z axis (vertical) datum and accordingly automatically adjust a Z axis movement device. For articulating robots, each joint is moved to correlate Z axis changes in relation to the fixed base. A more traditional capacitive sensor has also been used to sense the distance between a welding head and the workpiece material. Such a capacitive sensor is disclosed within U.S. Pat. No. 5,428,280 entitled "Robotic Movement of Object over a Workpiece Surface" which issued to Schmidt et al. on Jun. 27, 1995, and is incorporated by reference herewithin.
Laser welding and cuffing with a CO.sub.2 laser or a Yag laser are also becoming commonplace. Laser welding is highly advantageous over other types of welding methods since laser welding allows for deep and high speed welding without requiring the difficult to handle and somewhat costly filler material. Furthermore, laser welding devices are significantly less expensive as compared to other types of welding equipment. However, traditional laser welding and cutting systems use indexed turning or steering mirrors to redirect the laser beam along a curved workpiece or seam. Such mirrored systems are disclosed within the following U.S. Pat. Nos.: 4,972,062; 4,855,564; 4,677,274; and 4,367,017. The necessity to rotate these types of mirrors about the laser beam requires complicated and costly computer programming while also being somewhat prone to damage in the workpiece environment. Furthermore, such redirecting mirrors also provide additional tolerance inaccuracies and tend to collect airborne debris. Additionally, a fixed pivot point positioned on or above the laser in traditional systems causes focal length and focal point inaccuracies in relation to the workpiece material when the laser is initially oriented or moved relative to the workpiece surface; this is the scenario disclosed in U.S. Pat. No. 5,190,204.
Moreover, a large pre-weld gap between adjacent sheet material edges typically prevents adequate laser butt welding. This problem is usually observed with pre-weld gaps having a material edge-to-edge dimension greater than 10 percent of the material thickness. The welding machine is shut off and the material is scrapped if the maximum gap width is present. Such a gap problem is recognized in U.S. Pat. No. 5,204,505. Accordingly, it would be desirable to provide a laser welding apparatus and control system employing an improved, accurate and automated mechanism for welding across a large seam gap.